loss of oil
 

Does anyone know how to remove the black lid?

Use a screw and puller.

So I have to break it?
pushing from the valve plug could be removed?

If you can get something in there, thinking you bought new and was replacing?

I would like to take that out and mount it again by sealing it

No, buy new! Old rubber is nothing to save, its 5dollars.

Exactly as XTROCK says , rip it out and replace with new cap that has a good rubber coating to seal properly . I don't remember if a person can get at it good enough from behind to pop it out without distorting it even . But then the rocker cover has to come off and be resealed as well , rather than simply rapping on a sharp awl to punch a hole in the cover and pry out , then tap the new one back in (squarely) with no need to remove the cover.